The interaction of a minor protein component of the Escherichia coli outer membrane with a major porin protein to form an active colicin receptor has been demonstrated. This may represent a common mechanism whereby minor protein components of the outer membrane (each with a specific transport function) interact with major proteins (each with a non-specific transport function) to accomplish selected functions. These functions for example, include promoting the facilitated diffusion of hydrophilic substances such as vitamin B12 or ferrichrome across the outer membrane.